Novel multi-criteria aero-structural framework for sustainable UAV wings using PLA and epoxy biocomposites

ABSTRACT

This study introduces a novel multi-criteria decision-making (MCDM) framework that uniquely integrates aerodynamic gust loading, turbulence intensity, and fiber-matrix interfacial adhesion to select sustainable composites for unmanned aerial vehicle (UAV) wings. The primary objective is to identify optimal biodegradable and conventional materials that balance structural performance, environmental sustainability, and aerodynamic resilience under harsh desert-coastal wind conditions. Using advanced micromechanical models and a two-step MCDM methodology, natural fibers (ramie, flax, hemp, banana) reinforced with epoxy and polylactic acid (PLA) matrices were systematically evaluated against E-glass composites. Properties were normalized for density, strength, and modulus, followed by dynamic gust-induced sensitivity analysis incorporating realistic adhesion coefficients. Results demonstrate that ramie/epoxy achieves the highest overall score (0.855), excelling in specific stiffness and flutter resistance, while flax/epoxy delivers the most balanced structural profile (0.811). For fully biodegradable applications, ramie/PLA emerges as the optimal candidate (0.759), exhibiting the lowest performance degradation under extreme gust loading (49.1%). The study establishes a robust sustainable material selection framework for lightweight UAV structures and provides practical guidance for future biodegradable aerospace composite design, aeroelastic optimization, and environmentally responsible UAV manufacturing.

Keywords:
UAV Wings; Cellulosic Fibers; E-glass; PLA; Epoxy; Composite Material; Sustainable Materials.

1. INTRODUCTION

UAV airframes are subjected to high stress and severe operating conditions, including wind loads, lift and drag forces, ultraviolet radiation, and repeated fatigue loading. For this reason, UAV wings are commonly manufactured from synthetic composite materials. Over time, these materials suffer damage and degradation, and once the wings reach the end of their service life, they generate non-recyclable waste. One solution proposed in this study is the use of biodegradable materials for UAV wing fabrication. Natural Fiber-Reinforced Polymers (NFRPs) offer a promising alternative that can reduce non-recyclable material waste at the end of the UAV life cycle.

Natural fibers and NFRPs have demonstrated sustainable performance in many applications, responding to increasing environmental concerns and the global push toward greener energy and manufacturing practices [¹]. These composites use cellulosic fibers such as palm, alfa, sisal, jute, kenaf, and flax as reinforcement in polymer matrices, providing a viable substitute for conventional synthetic composites [², ³].

Their growing adoption is driven by several factors, including stricter environmental regulations, instability in petroleum markets, and increased environmental awareness at both societal and industrial levels [⁴, ⁵]. The performance and commercial viability of natural fiber composites depend mainly on the mechanical, physical, and recyclability characteristics of the fibers, which are essential for achieving cost-effective and reliable designs [⁵]. In modern engineering design, balancing material performance, environmental impact, and end-of-life recyclability has become a key requirement [⁶, ⁷]. However, material selection remains a complex multi-criteria decision-making problem, influenced by competing technical, economic, and environmental constraints [⁸, ⁹]. Addressing these challenges requires advanced selection approaches such as optimization methods and decision models to identify the most suitable material [¹⁰, ¹¹].

Efficient use of natural resources is central to sustainable engineering. The use of natural fibers, often sourced from agricultural waste, offers dual benefits by reducing waste disposal issues and lowering the environmental impact associated with synthetic fiber production [¹², ¹³]. Due to these advantages, natural fiber composites have shown strong potential to replace glass or carbon fiber composites in many semi-structural and structural applications [¹⁴]. Their benefits include low cost, low density, high specific properties, improved energy recovery, good thermal performance, and a reduced carbon footprint through CO₂ sequestration [¹⁵,¹⁶,¹⁷].

The properties and final performance of components made from natural fiber composites depend not only on the individual properties of the fibers and matrix but also on the quality of interfacial bonding and compatibility between them. This interaction plays a critical role in tailoring material properties for specific applications [¹⁸, ¹⁹].

Therefore, chemical treatments such as alkalization, acetylation, and silane treatment are necessary to remove undesirable constituents including hemicellulose, lignin, pectin, waxes, and surface impurities. These components negatively affect fiber-matrix adhesion and moisture resistance [¹⁹,²⁰,²¹]. Studies have investigated polypropylene (PP) and polylactic acid (PLA) composites reinforced with alfa cellulose microfibers [¹⁹], as well as PP-hemp composites processed by injection molding with emphasis on fiber treatment and quality optimization [²⁰]. Earlier work on jute-based plastic composites examined crystallinity, thermal behavior, weathering resistance, and suitability for eco-designed automotive components [²¹].

Additional research has explored the use of date palm fibers as reinforcement materials [²²] and evaluated date palm midribs for wood-cement composite applications [²³]. Broader studies have also compared bio-based plastics with petrochemical alternatives, focusing on environmental performance, market potential, and policy implications [²⁴].

The aerodynamic performance of small unmanned aerial vehicle (UAV) wings is strongly influenced by operation in the low Reynolds number regime. Previous studies investigated UAV wing geometry effects [²⁵], aerodynamic parameter estimation for small fixed-wing UAVs [²⁶], atmospheric conditions along the Saudi Red Sea coast [²⁷], wind characteristics in the Neom region [²⁸], and the suitability of micromechanical prediction methods for composite materials [²⁹]. Fiber-matrix adhesion effects were also modeled to improve composite property prediction [³⁰]. Reviews and experimental studies further examined natural fiber composites, sustainable applications, and surface treatments for better performance [³¹,³²,³³,³⁴]. Comparative studies between PLA and bio-epoxy composites provided insight into biodegradability and mechanical trade-offs [³⁵]. Fatigue behavior under cyclic loading was evaluated for flax/epoxy composites [³⁶] and flax/PLA composites [³⁷], while impact and processing effects in PLA biocomposites were reported in later studies [³⁸, ³⁹]. Mechanical and acoustic properties of flax/epoxy laminates were also investigated [⁴⁰]. Environmental aging due to UV and weather exposure was studied for epoxy composites [⁴¹] and PLA-based composites [⁴²].

Additional aerodynamic studies analyzed fixed-wing UAV swarm flight [⁴³], delta-wing UAV models [⁴⁴], and CFD behavior of long-endurance UAVs [⁴⁵]. Wind forecast data for Neom were also considered [⁴⁶]. Gust loading behavior was examined through wind direction and height effects [⁴⁷], tower-based gust measurements [⁴⁸], and reviews of gust factors and structural response methods used in standards [⁴⁹]. Further work reviewed impact behavior of natural fiber epoxy composites [⁵⁰], sustainable natural fiber reinforced PLA composites [⁵¹], epoxy/synthetic fiber fabrication methods [⁵²], and glass fiber reinforced PLA composites with improved mechanical performance [⁵³]. To account for multiple design and material selection criteria for UAVs [⁵⁴], MCDM has become one of the most recent and effective methods [⁵⁵].

This study places environmental and sustainability criteria at the center of selecting a biodegradable polymer airframe for UAV applications reinforced with natural cellulosic fibers. It compares several natural fibers with conventional glass fiber reinforcements to identify the most suitable natural fiber candidates for use with PLA or epoxy resin. The selected materials will support future experimental work focused on the design, fabrication, and testing of biodegradable UAV wings.

2. MATERIALS AND METHODS

The aerodynamic forces acting on small Unmanned Aerial Vehicle (UAV) wing are fundamentally governed by the interplay between pressure distribution and viscous effects, with the primary lifting force generated through controlled airflow manipulation around the wing’s airfoil profile, Figure 1 shows the different forces and moments in UAV. At the core of this phenomenon lies Bernoulli’s principle coupled with Newton’s third law, where the wing’s shape accelerates airflow over its upper surface, creating a pressure differential that produces lift. For a small UAV operating at relatively low Reynolds numbers of Re = 2.8 × 10⁵ [²⁵] or Re = 2 × 10⁵ [²⁶], due to smaller dimensions and speeds.

Figure 1
UAV forces and moments (4540 mm × 1200 mm).

The total forces on a UAV wing thus represents a delicate balance between aerodynamic efficiency, stability requirements, and control authority, all significantly influenced by the low Reynolds number regime in which most UAVs operate, necessitating specialized design approaches distinct from manned aviation.

To conduct simulation in UAV structure the region of Tabuk and Neom in Saudia arabia was taken as reference.

Tabuk city show average yearly wind speed of 5.5 m/s [²⁷] and in Neom region the wind speed is between 3 to 8.25 m/s [²⁸].

2.1. The UAV composite models

For a unidirectional natural fiber composite, the rules of mixture are used to estimate composite properties from the properties of the fiber and the matrix. The fibers are assumed to be continuous, aligned, and well bonded to the matrix. Perfect interfacial bonding is considered. Uniform stress or strain conditions are assumed depending on the loading direction. The material is assumed to be free of voids and defects. The fiber volume fraction is approximately 50%. The elastic response of the composite is described by the following equations [¹⁹].

Volume fractions include fiber volume fraction V_f and matrix volume fraction Vm.

(1) $${\text{V}}_{\text{f}}{\text{+ V}}_{\text{m}}\text{= 1}$$

Longitudinal modulus applies to loading parallel to the fibers. An iso strain condition is assumed. Fiber and matrix experience the same strain.

(2) $${\text{E}}_{\text{c}}{\text{= E}}_{\text{f}}{\text{V}}_{\text{f}}{\text{+ E}}_{\text{m}}{\text{V}}_{\text{m}}$$

This represents the upper bound. It is used to estimate stiffness along the fiber direction. Transverse modulus applies to loading perpendicular to the fibers. An iso stress condition is assumed. Fiber and matrix carry the same stress.

(3) $${\text{1/E}}_{\text{c}}{\text{= V}}_{\text{f}}{\text{/ E}}_{\text{f}}{\text{+ V}}_{\text{m}}{\text{/ E}}_{\text{m}}$$

Tensile strength in the fiber direction is estimated as

(4) $${\sigma }_{\text{c}}\approx {\sigma }_{\text{f}}{\text{V}}_{\text{f}}\text{+}{\sigma }_{\text{m}}{\text{V}}_{\text{m}}$$

Composite density is given by:

(5) $${\text{ρ}}_{\text{c}}{\text{= ρ}}_{\text{f}}{\text{V}}_{\text{f}}{\text{+ ρ}}_{\text{m}}{\text{V}}_{\text{m}}$$

Poisson’s ratio follows

(6) $${\text{ν}}_{\text{c}}{\text{= ν}}_{\text{f}}{\text{V}}_{\text{f}}{\text{+ ν}}_{\text{m}}{\text{V}}_{\text{m}}$$

Shear modulus in the fiber direction is estimated by

(7) $${\text{1/G}}_{\text{c}}{\text{= V}}_{\text{f}}{\text{/ G}}_{\text{f}}{\text{+ V}}_{\text{m}}{\text{/ G}}_{\text{m}}$$

The ability of different micromechanical models to predict the tensile modulus of composites was evaluated by JARIWALA and JAIN [²⁹]. Among the models considered, the Halpin-Tsai model showed the lowest total error. The corresponding equations are given below [²⁹].

(8) $${\text{E}}_{\text{c}}{\text{= E}}_{\text{m}}{\text{(1 + ξ η V}}_{\text{f}}{\text{) / (1 - η V}}_{\text{f}}\text{)}$$

Where:

(9) $${\text{η = (E}}_{\text{f}}{\text{/ E}}_{\text{m}}{\text{− 1) / (E}}_{\text{f}}{\text{/ E}}_{\text{m}}\text{+ ξ)}$$

where, the parameter ξ is defined as

(10) $$\xi \text{= 2 (L / T) or}\xi \text{= 2 (L / D)}$$

where L represents the fiber length along the loading direction, T the fiber thickness and D the fiber diameter.

2.2. The matrix - fiber adhesion effect on composite behavior

Matrix-fiber adhesion plays a central role in determining composite behavior. To represent realistic defects and variations in bonding quality, PAPANICOLAOU et al. [³⁰] proposed modified equations using an adhesion coefficient k_adh, as defined in Equation 11:

(11) $${\mathrm{k}}_{\mathrm{a}\text{d}\mathrm{h}}=\frac{{\mathrm{E}}_{\text{i}}\left({\mathrm{r}}_{\text{f}},\mathrm{t}\right)}{{\mathrm{E}}_{\text{f}}}$$

Were, the adhesion coefficient 0 < k_adh < 1

k_adh = 1: perfect adhesion (full stress transfer), k_adh = 0: no stress transfer

t: time

r_f: fiber radius

E_i: elastic modulus of the interphase

The effective fiber volume fraction is then expressed as shown in Equation 12.

(12) $${\text{V}}_{\text{eff}}{\text{= k}}_{\text{adh}}{\text{V}}_{\text{f}}$$

The longitudinal and transverse composite moduli, E_CL and E_CT , include the modulus of a single-fiber representative volume element (RVE), as given in Equations 13 and 14 [³⁰].

(13) $${\text{E}}_{\text{CL}}{\text{= E}}_{\text{f}}{\text{V}}_{\text{eff}}{\text{+ E}}_{\text{m}}{\text{(1 - V}}_{\text{eff}}\text{)}$$

(14) $${\mathrm{E}}_{\mathrm{C}\text{T}}=\frac{{\mathrm{E}}_{\text{f}}{\mathrm{E}}_{\text{m}}}{{\mathrm{E}}_{\text{m}}{\mathrm{V}}_{\mathrm{e}\text{f}\mathrm{f}}+{\mathrm{E}}_{\text{f}}\left(1-{\mathrm{V}}_{\mathrm{e}\text{f}\mathrm{f}}\right)}$$

The Anisotropy coefficient as shown in Equation 15.

(15) $${\mathrm{S}}_{\text{E}}=\frac{{\mathrm{E}}_{\text{T}}}{{\mathrm{E}}_{\text{L}}}$$

The interphase thickness as shown in Equation 16.

(16) $$\mathrm{\Delta }{\mathrm{r}}_{\text{i}}=-\frac{\left(1-{\mathrm{k}}_{\mathrm{a}\text{d}\mathrm{h}}\right).{\mathrm{S}}_{\text{g}}.{\mathrm{r}}_{\text{f}}}{{\mathrm{k}}_{\mathrm{a}\text{d}\mathrm{h}}}.\mathrm{I}\text{n}\left[\frac{{10}^{-3}}{{\mathrm{k}}_{\mathrm{a}\text{d}\mathrm{h}}{\mathrm{E}}_{\text{f}}-{\mathrm{E}}_{\text{m}}\left(\mathrm{t}\right)}\right]$$

Fiber-matrix debonding can be modeled through the polymer matrix, where the variation in elastic modulus within the hybrid interphase region is expressed in Equation 17 [³⁰].

(17) $${\mathrm{E}}_{\text{i}}\left(\mathrm{r},\mathrm{t}\right)={\mathrm{E}}_{\text{m}}\left(\mathrm{t}\right)+\left({\mathrm{k}}_{\mathrm{a}\text{d}\mathrm{h}}{\mathrm{E}}_{\text{f}}\left(\mathrm{t}\right)-{\mathrm{E}}_{\text{m}}\left(\mathrm{t}\right)\right).\exp \left(\left(-\frac{{\mathrm{k}}_{\mathrm{a}\text{d}\mathrm{h}}}{1-{\mathrm{k}}_{\mathrm{a}\text{d}\mathrm{h}}}\right)\frac{{\mathrm{E}}_{\mathrm{C}\text{L}}}{{\mathrm{E}}_{\mathrm{C}\text{T}}}\frac{\mathrm{r}-{\mathrm{r}}_{\text{f}}}{{\mathrm{r}}_{\text{f}}}\right)$$

Where:

r: radius within the hybrid interphase region (r_f ≤ r ≤ r_f + Δ r_i)

E_m and E_f vary with time and environmental conditions, which may include moisture absorption and UV radiation, especially in natural fibers and biodegradable polymer matrices.

The selection of the matrix polymer, whether thermoset epoxy or biodegradable PLA, strongly influences fiber-matrix adhesion behavior, which is quantified by the adhesion coefficient k_adh in Equation 11. For natural fiber composites, epoxy promotes better chemical compatibility and improved wetting of the hydrophilic fiber surface, which typically results in higher k_adh values. In contrast, PLA is relatively hydrophobic, creating an inherent polarity mismatch that leads to weaker interfacial bonding and lower k_adh values unless surface treatment or compatibilization is applied.

This difference directly affects the effective fiber volume fraction V_eff in Equation 12. For the same nominal fiber content, an epoxy-based composite can therefore utilize a greater effective fiber fraction than a PLA-based composite. As a result, the longitudinal and transverse elastic moduli, E_CL and E_CT, predicted by Equations 13 and 14, are generally higher for epoxy composites, as is the anisotropy coefficient S_E in Equation 15.

The interphase thickness Δr_i, defined in Equation 16, is also influenced by adhesion quality. Lower k_adh values, which may arise from moisture absorption, UV exposure, or manufacturing defects, increase the magnitude of Δr_i, indicating a thicker and more compliant interphase region. Accordingly, the radial modulus variation within the interphase, described by Equation 17, reflects more efficient stress transfer from the fiber surface to the epoxy matrix due to higher k_adh, whereas PLA-based composites typically exhibit weaker stress transfer efficiency.

Table 1 proposes an assessment of Compatibility of different fiber-matrix composites.

Table 1
The fiber-matrix compatibility.

FIBER-MATRIX COMPOSITE	COMPATIBILITY	REFERENCES
Natural fiber + Epoxy	Epoxy is polar and compatible with hydrophilic natural fibers	SEID and ADIMASS [50]
Natural fiber + PLA	PLA is relatively hydrophobic, causing polarity mismatch	ZHAO et al. [51]
Synthetic fiber + Epoxy	Excellent wetting and strong chemical bonding	PADMARAJ [52]
Synthetic fiber + PLA	Limited compatibility between glass fiber and PLA	WANG et al. [53]

2.3. Aerodynamic pressure effect on the UAV composite

During flight, UAV wings are continuously subjected to aerodynamic forces generated by airflow. These forces become more critical under gust wind conditions, where short-term increases in wind speed can significantly raise the pressure acting on the wing surface. For lightweight composite UAV structures, repeated gust loading may increase bending stress, matrix cracking, fiber-matrix debonding, fatigue damage, and local buckling risk. Therefore, evaluating wind-induced pressure effects is important when selecting composite materials such as epoxy- or PLA-based laminates for operation in desert regions such as Tabuk and Neom, where sudden wind fluctuations may occur. The aerodynamic load acting on the UAV wing can be estimated using dynamic pressure theory. The wind dynamic pressure is given by the standard aerodynamic relation is shown in Equation 18 [⁴³,⁴⁴,⁴⁵]:

(18) $$q=\frac{1}{\text{2}}{\rho }_{air}{V}^2$$

Where:

• q = dynamic pressure (Pa)

• ρ_air = 1.225 kg/m³ at sea level and 15°C

• V = wind speed (m/s)

The peak gust speed in Tabuk and Neom is in Neom and it is about 12 m/s [⁴⁶].

Assuming linear elastic behavior and that aerodynamic loads are proportional to dynamic pressure, the gust-induced stress is:

(19) $${\text{σ}}_{\text{gust}}{\text{= σ}}_{\text{ref}}\text{. LF}$$

Where:

• σ_ref = reference stress at mean wind condition (MPa)

• σ_gust = stress under gust condition (MPa)

The the dynamic Gust Load Factor can be calculated by the ratio of gust-induced dynamic pressure to mean dynamic pressure which can be written as in the Equation 19 [⁴⁷, ⁴⁸].

(20) $$LF=\frac{q_{gust}}{q_{mean}}=\frac{\frac{1}{\text{2}}{\rho }_{air}{V}_{gust}^2}{\frac{1}{\text{2}}{\rho }_{air}{V}_{mean}^2}$$

For Gaussian turbulence, the expected peak gust speed over a short duration (3–5 seconds) [⁴⁹] is:

(21) $${\text{V}}_{\text{peak}}\approx {\text{V}}_{\text{mean}}\text{(1 + 2}\text{. TI)}$$

Where: TI Turbulence intensity represents the standard deviation of wind speed fluctuations normalized by the mean wind speed.

The peak gust factor accounting for turbulence is:

(22) $${\text{CF}}_{\text{turb}}\text{= 1 + 2}\text{. TI}$$

The dynamic safety factor incorporates both gust loading and turbulence intensity:

(23) $${\text{SFdynamic = SF}}_{\text{base}}\text{× (1 + 2 · TI)}$$

Where: SF_base: epoxy/PLA based composites.

3. RESULTS

3.1. Performance of natural fibers versus synthetic fibers for UAV wing composite materials

The performance of reinforcing fibers is a key factor in the structural design of UAV wings, as it directly influences stiffness, strength, fatigue resistance, and overall flight safety.

A comparison between common natural fibers and conventional synthetic fiber, glass fiber reveals a clear difference in absolute mechanical properties, Table 1. To address the variability of natural fibers, a conservative design methodology was adopted. The material properties presented in Table 1 are based on mean values reported in peer-reviewed literature.

Table 2 shows various properties of natural fibers, Glass fiber, two polymer matrices and natural fiber composites. The observed results data in Tables 2, 3 and 4 vary considerably depending on chemical composition and structure, growing conditions, harvesting time, extraction method, treatment, and storage procedure [²⁹].

Table 2
Natural and synthetic fibers for UAV [²⁹,³⁰,³¹,³²].

FIBER	TENSILE STRENGTH (MPa)	DENSITY (g/cm3)	YOUNG’S MODULUS (GPa)
E-Glass	250 ± 70	2.50	72 ± 5
Kenaf	707 ± 242	1.05 ± 0.23	35.5 ± 12.3
Bamboo	537.5 ± 18.8	1.35 ± 0.08	33.5 ± 3.3
Hemp	1010 ± 362.5	1.35 ± 0.13	29.2 ± 7.4
Bagasse	275 ± 12.5	0.9 ± 0.05	18.5 ± 0.8
Ramie	669 ± 134.5	1.45 ± 0.03	94.5 ± 16.8
Flax	970 ± 315	1.4 ± 0.05	53.0 ± 14.0
Sugarcane	260 ± 45	1.35 ± 0.13	5.7 ± 0.3
Jute	583 ± 95	1.4 ± 0.05	19.8 ± 3.4
Oil palm	225 ± 87.5	1.15 ± 0.23	4.8 ± 2.1
Cotton	543.5 ± 128.3	1.55 ± 0.03	9.1 ± 1.8
Sisal	554 ± 43	1.45 ± 0.08	15.7 ± 3.2
Banana	750 ± 19.5	1.0 ± 0.25	18.4 ± 7.2
Coir	200 ± 15	1.4 ± 0.1	5.0 ± 1.0

Table 3
Matrices for construction of UAV wing

POLYMER	DENSITY (g/cm3)	TENSILE STRENGTH (MPa)	TENSILE MODULUS (GPa)	ELONGATION AT BREAK (%)	REFERENCES
Epoxy	1.175	88.0 ± 7.07	32.0 ± 1.41	4.9 ± 0.99	[32, 33]
PLA	1.29	37.0 ± 0.0	9.35 ± 9.41	25.0 ± 7.07	[33]

Table 4
Assumed criteria’s weights.

CRITERION	WEIGHT	JUSTIFICATION
Density wρ	0.3	UAVs require lightweight materials for flight efficiency
Strength wσ	0.3	Must withstand aerodynamic and maneuver loads
Modulus wE	0.4	Wing stiffness critical for preventing flutter and maintaining aerodynamic shape

3.2. Data normalization for the multi criteria decision making (MCDM) of the UAV composite

Since different properties have different units and scales, normalization is required to make them comparable (all scaled to 0-1 range).

For Strength, Modulus, Specific Properties higher values is better because is gives higher strength for the UAV

(24) $$x_{norm}=\frac{x-x_{min}}{x_{max}-x_{min}}$$

Where:

x_norm = Normalized value (0 to 1)

x = Original value

x_max= Maximum value in the dataset

x_min = Minimum value in the dataset

For the Density lower values is better because that gives less weight for the UAV

Method 1 - Reciprocal:

(25) $$x_{norm}=\frac{1/x}{max\left(1/x\right)}$$

Method 2 - Linear Transformation:

(26) $$x_{norm}=1-\frac{x-x_{min}}{x_{max}-x_{min}}$$

Both methods Equations 11 and 12, ensure that lower density results in higher normalized scores.

The final MCDM score is calculated using a weighted linear combination, Equation 21.

(27) $$S_j={\displaystyle \sum {}_{i=1}^n}{w}_i.x_{ij}^{norm}$$

S_j = Final score for composite

w_i = Weight assigned to criterion

$x_{ij}^{norm}$ = Normalized value of composite

n = Number of criteria

The composite analysis was conducted based in the Equation 21 and the weights presented in the Table 4.

(28) $$S_j=w_{\rho }\times {\rho }_j^{norm}+w_{\sigma }\times {\sigma }_j^{norm}+w_E\times E_j^{norm}$$

The Multi-Criteria Decision-Making final scores for 50% fiber Composites for the best nine composites with matrix Epoxy/PLA in mixture with 50% weight of fibers.

To check the robustness of the ranking, sensitivity analysis can be performed by varying weights:

(29) $$S_j\left(\mathrm{\Delta }w\right)=\left(w_{\rho }\pm \mathrm{\Delta }w\right){\rho }_j^{norm}+\left(w_{\sigma }\pm \mathrm{\Delta }w\right){\sigma }_j^{norm}+\left(w_{{\rm E}}\mp 2\text{Δ}w\right)E_j^{norm}$$

Where Δw is the weight variation (0.05 to 0.1).

The Figure 2 shows the Multi-Criteria Decision-Making final scores for 50% fiber Composites for the best ten composites with matrix Epoxy/PLA in mixture with 50% weight of E-Glass/ Ramie/ Flax/ Hemp/ Banana.

Figure 2
Multi-criteria decision-making scores for 50% fiber composites.

Figure 3 shows the performance profile of the top five composites of the MCDM analysis.

Figure 3
Performance profile of the top five composites of the MCDM analysis.

Figure 2 presents the MCDM ranking of the top ten candidate materials for UAV wing structures. The results establish a clear performance order. Ramie/Epoxy holds the first position with a score of 0.855. It exhibits the highest modulus in the dataset at 63.3 GPa, which makes it well suited for high aspect ratio wings where structural rigidity is required to limit flutter. Its leading position is mainly driven by stiffness related criteria. Flax/Epoxy ranks second with a score of 0.835. It combines high tensile strength of 529 MPa with a modulus of 42.5 GPa. Its balanced mechanical profile supports its selection as a versatile material for general UAV wing applications. Hemp/Epoxy occupies the third position with a score of 0.815. It achieves the highest tensile strength at 549 MPa, which favors its use in primary load carrying components such as wing spars. Although its modulus is lower than Ramie/Epoxy, its strength based performance remains competitive.

Epoxy based composites dominate the ranking. They occupy the top six positions. They outperform PLA based composites due to higher stiffness and strength. Among biodegradable options, Hemp/PLA leads with a score of 0.759. It maintains good tensile strength at 523.5 MPa but shows reduced stiffness at 19.3 GPa.

Flax/PLA follows at 0.752. It provides the most balanced, fully biodegradable solution. E-Glass composites rank last. E-Glass/Epoxy scores 0.655. E-Glass/PLA drops to 0.588. Both suffer from high density around 1.84 to 1.90 g/cm³ and weaker strength to weight performance. They do not meet lightweight UAV design demands. There is a clear sustainability trade off. Selecting Flax/PLA instead of Ramie/Epoxy reduces the performance score by 0.103 points. Material selection depends on design priorities. Choose Ramie/Epoxy for maximum stiffness and structural performance. Choose Flax/Epoxy for balanced mechanical behavior. Choose Flax/PLA for full biodegradability with moderate performance. And avoid E-Glass composites for lightweight UAV wings.

Figure 3 presents a radar chart comparing the top five composites across five normalized criteria. Ramie/Epoxy shows a profile stretched toward Modulus and Specific Modulus. It achieves near maximum normalized values in stiffness related metrics. This confirms its suitability for flutter critical wing designs. However, its strength values are slightly lower than Hemp/Epoxy. Flax/Epoxy displays the most uniform polygon. It maintains consistently high scores across all five criteria. This visual balance confirms its role as the safest all round structural material. Hemp/Epoxy extends further along the Strength and Specific Strength axes. Its tensile strength of 549 MPa drives this shape. Its modulus of 30.6 GPa causes a slight inward pull along the stiffness axis. PLA based composites show smaller polygons. This reflects the lower mechanical capacity of the biodegradable matrix. Ramie/PLA retains strong stiffness at 51.9 GPa but drops in strength to 353 MPa. Flax/PLA maintains better balance with 503.5 MPa strength and 31.2 GPa modulus. It stands as the strongest fully biodegradable candidate. The radar chart highlights the core design conflict. Ramie/Epoxy maximizes stiffness. Flax/Epoxy balances stiffness and strength. PLA composites reduce mechanical performance to gain sustainability.

3.3. Gust wind factor influence the MCDM ranking

To assess how material rankings change under gust loading, a penalty factor is applied to the MCDM score based on the adhesion coefficient:

(30) $${\text{P}}_{\text{GUST}}\text{= exp (-}\alpha {\text{. k}}_{\text{ADH}}\text{. (G - 1))}$$

Where:

P_GUST = Gust Penalty Factor

k_ADH = Adhesion Coefficient

G = Gust Factor

α = Sensitivity Coefficient

The critical flutter speed for a wing is proportional to the square root of the stiffness-to-mass ratio, as in the equation:

(31) $$V_{Flutter}\alpha \sqrt{\frac{E_{\mathrm{CI}}}{{\rho }_C}}$$

Where:

E_CL = Longitudinal Composite Modulus (Gpa)

ρ_c = Composite Density (Kg/M³)

The lift force on the wing under gust conditions scales with dynamic pressure and angle of attack:

(32) $$L_{\mathrm{GUST}}=\frac{1}{\text{2}}{\rho }_{Air}{V}_{\mathrm{Gust}}^2.S_{\mathrm{Wing}}.C_L\left(\alpha \right)$$

Where:

L_GUST = Lift Force Under Gust

S_WING = Wing Planform Area

C_L(α) = Lift Coefficient as Function of Angle of Attack

Composite Stress Under Combined Loading

For A Wing Subjected to Bending Moment from Lift Forces, The Maximum Stress in the Wing Skin Is:

(33) $${\sigma }_{Max}=\frac{M.y}{I}$$

Where:

M = Bending Moment

y = Distance From Neutral Axis to Outer Fiber

I = Area Moment of Inertia

3.4. The MCDM in gust wind conditions

The base Weighted linear combination of normalized properties can express the Base MCDM as shown in the Equation 34.

(34) $$Base=w_{\rho }.{\rho }_{norm}+w_{\sigma }.{\sigma }_{norm}+w_{{\rm E}}.{{\rm E}}_{norm}$$

Where:

(35) $${\rho }_{norm}=1-\frac{\rho -{\rho }_{min}}{{\rho }_{max}-{\rho }_{min}}$$

(36) $${\sigma }_{norm}=\frac{\sigma -{\sigma }_{min}}{{\sigma }_{max}-{\sigma }_{min}}$$

(37) $${{\rm E}}_{norm}=\frac{{\rm E}-{{\rm E}}_{min}}{{{\rm E}}_{max}-{{\rm E}}_{min}}$$

The MCDM score at mean wind condition (no gust, low turbulence) can be expressed as in the Equation 38:

(38) $$MCD{M}_{\mathrm{mean}}=\mathrm{Base}\times P_{\mathrm{turb}}(10\%)$$

Where:

P_turb (TI) = exp (–0.5 × TI)

TI = 0.10 (10% turbulence intensity)

G = 1: no gust wind

The MCDM score at moderate gust condition

(39) $$MCD{M}_{\mathrm{Moderate}}=\mathrm{Base}\times P_{\mathrm{gust}}(G=1.5)\times P_{\mathrm{turb}}(15\%)$$

Where:

(40) $$P_{\mathrm{gust}}(G)=\exp (-k_{adh}.(G-1))$$

MCDM score at severe gust condition (Neom peak gust + turbulence)

(41) $$MCD{M}_{\mathrm{Severe}}=\mathrm{Base}\times P_{\mathrm{gust}}(G=2.0)\times P_{\mathrm{turb}}(20\%)$$

MCDM score at extreme gust condition (worst-case scenario)

(42) $$MCD{M}_{\text{Extreme}}=Base\times P_{\text{gust}}(G=2.5)\times P_{\text{turb}}(25\%)$$

Two sensitivity metrics are calculated:

Absolute Sensitivity (Delta):

(43) $${\text{V}}_{\text{condition}}{\text{= MCDM}}_{\text{base}}{\text{- MCDM}}_{\text{aero}}$$

Relative Sensitivity Index:

(44) $$\mathrm{S}\text{I}=\frac{\mathrm{M}\text{C}\mathrm{D}{\mathrm{M}}_{\mathrm{m}\text{e}\mathrm{a}\text{n}}-\mathrm{M}\text{C}\mathrm{D}{\mathrm{M}}_{\mathrm{e}\text{x}\mathrm{t}\text{r}\mathrm{e}\text{m}\mathrm{e}}}{\mathrm{M}\text{C}\mathrm{D}{\mathrm{M}}_{\mathrm{m}\text{e}\mathrm{a}\text{n}}}\times 100\text{\%}$$

3.5. The MCDM results based on NACA 4412 airfoil

The peak gust speed in Tabuk and Neom is in Neom and it is about 12 m/s [⁴⁶]. The airfoil is choosen as NACA4412. It is known for its high lift capabilities and reliability, making it a good choice in aviation UAV wings.

Figure 4 shows the lift force versus wind speed for the NACA 4412 airfoil at angles of attack ranging from 0º to 14º. The lift force follows a quadratic relationship with wind speed, increasing from negligible values at 0º to approximately 280 N at 14º and 25 m/s. For Tabuk and Neom operating conditions (mean winds of 5.5-6.0 m/s and peak gusts of 12 m/s, the lift force necessitates careful material selection to ensure structural safety.

Figure 4
NACA 4412 Air foil performance, lift force vs wind speed.

Figure 5 illustrates the decay of MCDM scores with increasing gust factor G for all ten composite materials under constant turbulence (TI = 15%). All scores decrease exponentially as G increases from 1.0 to 3.0, following the penalty function governed by the adhesion coefficient k_adh. At mean wind (G = 1.0), Ramie/Epoxy leads with a score of 0.78, followed by Flax/Epoxy (0.68) and Banana/Epoxy (0.52). At severe gust conditions corresponding to Neom’s peak gust (G = 2.0), Ramie/Epoxy declines to 0.42, Flax/Epoxy to 0.37, and PLA-based materials fall below 0.30. Under extreme gusts (G = 2.5), the top-performing epoxy materials score between 0.22 and 0.32, while PLA materials range from 0.09 to 0.18. Despite the significant score reductions, the ranking order remains consistent across all gust levels: epoxy-based composites consistently outperform PLA-based composites, with Ramie/Epoxy maintaining the highest position and E-Glass/PLA the lowest, confirming the robustness of the material selection.

Figure 5
Influence of gust wind factor in MCDM Ranking.

Table 5 summarizes the sensitivity analysis for the top ten composite materials under progressively severe aerodynamic conditions. Ramie/Epoxy achieves the highest absolute scores at all conditions (base: 0.855, extreme: 0.257) with a moderate sensitivity index of 67.5%. Flax/Epoxy follows closely (base: 0.811, extreme: 0.220, sensitivity: 69.9%). Among PLA-based materials, Ramie/PLA demonstrates superior performance (base: 0.759, extreme: 0.340) and the lowest sensitivity index (49.1%), indicating excellent relative retention of performance. E-Glass/Epoxy exhibits the highest sensitivity index (74.0%), while E-Glass/PLA records the lowest absolute scores. The analysis confirms that epoxy-based materials offer higher absolute performance, while PLA-based materials, particularly Ramie/PLA, provide viable biodegradable alternatives with lower relative sensitivity to gust loading.

Table 5
Sensitivity analysis for the top 10 composite materials.

MATERIAL	MATRIX	BASE	MEAN	MODERATE	SEVERE	EXTREME	Δ MEAN	Δ EXT-REME	SENSITIVITY
Ramie/Epoxy	Epoxy	0.855	0.793	0.545	0.374	0.257	0.062	0.598	67.5%
Flax/Epoxy	Epoxy	0.811	0.732	0.491	0.329	0.220	0.079	0.591	69.9%
Ramie/PLA	PLA	0.759	0.668	0.533	0.426	0.340	0.091	0.419	49.1%
Hemp/Epoxy	Epoxy	0.752	0.690	0.470	0.320	0.218	0.062	0.534	68.4%
Flax/PLA	PLA	0.715	0.638	0.497	0.387	0.302	0.077	0.413	52.7%
Banana/ Epoxy	Epoxy	0.688	0.628	0.428	0.291	0.198	0.060	0.490	68.5%
Hemp/PLA	PLA	0.655	0.576	0.448	0.348	0.271	0.079	0.384	52.9%
E-Glass/Epoxy	Epoxy	0.599	0.542	0.346	0.221	0.141	0.057	0.458	74.0%
Banana/PLA	PLA	0.588	0.513	0.398	0.309	0.240	0.075	0.348	53.2%
E-Glass/PLA	PLA	0.508	0.450	0.359	0.287	0.229	0.058	0.279	49.1%

Table 6, presents the key properties of the five highest-ranked composite materials. Ramie/Epoxy achieves the highest base MCDM (0.855) and specific stiffness (43.7 GPa·cm³/g), making it optimal for flutter-critical applications. Flax/Epoxy offers the best balance of properties, with the highest adhesion coefficient (k_adh = 0.75), good strength (529 MPa), and a reference flutter ratio of 1.00. Ramie/PLA is the top biodegradable candidate, combining a high base MCDM (0.759) with excellent specific stiffness (37.1 GPa·cm³/g) despite a low adhesion coefficient (0.40). Hemp/Epoxy provides the highest tensile strength (549 MPa) but the lowest specific stiffness among the top five. Flax/PLA represents a balanced biodegradable alternative with the highest k_adh among PLA composites (0.45) and competitive strength (503.5 MPa). Epoxy-based materials consistently outperform PLA-based composites in mechanical properties, while PLA composites offer the advantage of full biodegradability.

Table 6
Results on the top 5 composites.

MATERIAL	MATRIX	BASE MCDM	STRENGTH (MPa)	SPEC. STIFF.	k_adh	FLUTTER RATIO	BIODEGRADABLE
Ramie/Epoxy	Epoxy	0.855	353	43.7	0.70	1.20	No
Flax/Epoxy	Epoxy	0.811	529	30.4	0.75	1.00	No
Ramie/PLA	PLA	0.759	353	37.1	0.40	1.10	Yes
Hemp/Epoxy	Epoxy	0.752	549	22.7	0.72	0.86	No
Flax/PLA	PLA	0.715	503.5	23.1	0.45	0.87	Yes

Figure 6 displays a radar chart comparing the top five composite materials across five performance criteria. Ramie/Epoxy exhibits the largest profile, excelling in specific stiffness (0.95), base MCDM (0.50), and flutter margin (0.20). Flax/Epoxy shows the most balanced polygon, with high scores across all categories, reinforcing its status as the optimal all-round material. Hemp/Epoxy is distinguished by its highest tensile strength score (0.90), making it suitable for load-bearing applications. Among biodegradable options, Ramie/PLA demonstrates a surprisingly large profile, outperforming Hemp/Epoxy in specific stiffness (0.80) and gust tolerance (0.09). Flax/PLA exhibits a more compact but balanced polygon. The chart visually confirms that epoxy materials occupy the outer performance envelope, while PLA materials form an inner cluster, illustrating the performance penalty associated with full biodegradability. However, Ramie/PLA demonstrates that carefully selected fiber-matrix combinations can narrow this gap considerably.

Figure 6
Performance profile of the top five composites taking into acount base MCDM, strength (MPa), specific stiffness, kadh, flutter ratio and biodegradability.

After taking in consideration different wind posible cases including Gust wind and turbilances the radar chart in the Figure 6, demonstrates that Flax/Epoxy has remain the most balanced performance profile (largest area), while Ramie/Epoxy excels in stiffness-related metrics, and Ramie/PLA leads among biodegradable options.

4. DISCUSSION

4.1. Influence of aerodynamic loading on the MCDM ranking

The aerodynamic loading in this study was based on average wind speeds in Tabuk 5.5 m/s and Neom 6.0 m/s, with peak gust of 12 m/s. However, wind conditions can shift rapidly to turbulent states, creating significant challenges for UAV structures, especially those reinforced with natural fibers. The sensitivity analysis revealed that as the gust factor increases from G = 1.0 to G = 2.5. Under severe gust conditions (G = 2.0, TI = 15%), Ramie/Epoxy maintains the highest MCDM score (0.581), followed by Flax/Epoxy (0.550) and Ramie/PLA (0.524). Under extreme gust conditions (G = 2.5, TI = 25%), these values decline to 0.257, 0.220, and 0.340 respectively. Notably, Ramie/PLA demonstrates superior retention of performance under extreme gusts compared to epoxy composites due to its lower sensitivity index. A conservative design approach is recommended: a dynamic safety factor of at least 3.0 should be used for epoxy-based composites, and 5.0 for PLA-based composites under turbulent operating conditions. Further validation is required through dynamic mechanical analysis and fatigue testing [⁵⁶], under representative gust spectra, coupled with CFD-FEA simulations of transient gust loads on the UAV wing.

The presence of moisture would further degrade stiffness and lower flutter speeds over time, with PLA-based composites being more severely affected due to hydrolytic degradation. For the present material selection, Ramie/Epoxy is recommended as the safest choice for flutter-critical UAV wing applications, offering the highest specific stiffness and greatest margin against aeroelastic instability. Flax/Epoxy provides a balanced alternative with sufficient flutter margins for most applications, while Ramie/PLA offers the best biodegradable option for flutter-resistant designs.

4.2. Composite behavior under compression, buckling, and shear loading

Under compressive loading, natural fibers such as ramie, flax, and hemp tend to undergo micro-buckling due to their lower lateral strength and irregular cross-sectional morphology, resulting in compressive strengths that are lower of their tensile strengths. In contrast, glass fibers, being isotropic and perfectly circular, demonstrate more balanced tension-compression behavior. For thin wing skins, buckling resistance is governed primarily by flexural stiffness, which depends on the composite’s modulus and laminate stacking sequence. Ramie/Epoxy, with a modulus of 63.3 GPa, provides superior buckling resistance compared to Flax/Epoxy (42.5 GPa) and Ramie/PLA (51.9 GPa), while Flax/PLA (31.2 GPa) reduces the critical buckling load for the same skin thickness. Regarding shear loading in the wing spar web, natural fiber composites exhibit lower interlaminar shear strength compared to glass due to weaker fiber-matrix adhesion. Additionally, the interphase thickness described in Equation 16 increases under shear-dominated loading as partial debonding propagates along the fiber length.

4.3. Influence of fatigue life, impact resistance, moisture diffusion rate, and thermal stability

The current MCDM model considers density, tensile strength, and modulus as primary criteria, which adequately captures static structural performance. However, the inclusion of fatigue life, impact resistance, moisture diffusion rate, and thermal stability would substantially alter the final ranking, particularly for PLA-based biodegradable composites. Natural fiber composites are inherently susceptible to moisture absorption due to fiber hydrophilicity, and PLA matrices exacerbate this issue through hydrolytic degradation, leading to accelerated property loss under cyclic humidity and UV radiation. Epoxy-based composite, while not biodegradable, offer superior moisture resistance and thermal stability, maintaining structural integrity under prolonged environmental exposure.

Under fatigue loading, PLA composites exhibit rapid stiffness degradation and interfacial debonding [³⁷], whereas flax/epoxy demonstrates excellent fatigue endurance [³⁶]. Regarding impact resistance, epoxy composites are brittle and susceptible to delamination, while PLA offers moderate toughness [³⁸,³⁹,⁴⁰]. The PLA matrix loses ductility under UV exposure more rapidly than epoxy [⁴¹, ⁴², ⁵⁷]. Incorporating these criteria would widen the performance gap between epoxy and PLA composites, further consolidating Ramie/Epoxy as the top-ranked material for stiffness-critical applications and Flax/Epoxy for balanced performance.

4.4. Influence of varying fiber percentage on the MCDM ranking

The fiber volume fraction affects both structural performance and environmental impact, while also influencing manufacturability, including the formation of resin-rich zones and the overall stiffness of the composite. In this study, the fiber volume fraction V_f was fixed at 50%, a value widely used for continuous fiber composites as it balances mechanical performance with ease of manufacturing. However, varying V_f between 30% and 60% leads to clear trade-offs that affect the ranking. Increasing V_f improves longitudinal stiffness E_CL: for Ramie/Epoxy, increasing from 50% to 60% would raise E_CL, improving flutter resistance but would risk manufacturability defects due to poor resin infiltration. For Flax/Epoxy, varying V_f from 50% to 55% would increase E_CL, raising the flutter margin. Reducing V_f to 40% lowers E_CL, which may still satisfy flutter constraints for moderate-aspect-ratio wings but reduces safety margins against turbulence-induced oscillations. For PLA-based composites, the optimal V_f is lower, around 40%, because higher fiber content exacerbates the already poor fiber-matrix adhesion, leading to void formation and reduced effective stress transfer. At V_f = 50% Ramie/PLA and Flax/PLA may exhibit significant performance degradation due to incomplete wetting, whereas at V_f = 40%, the PLA matrix can more effectively infiltrate fiber bundles, partially compensating for the lower k_adh value. The environmental trade-off is also notable: higher V_f increases renewable fiber content but may require additional processing energy, while lower V_f reduces the biodegradable matrix fraction.

4.5. Influence of realistic defects on the MCDM ranking

Real manufacturing processes inevitably introduce voids, material defects, partial fiber-matrix debonding, and porosity, particularly for natural fiber composites, which exhibit inherent variability in adhesion quality due to their hydrophilic nature, surface impurities, and irregular morphology. These realistic factors are quantitatively considered through the adhesion coefficient k_adh introduced in Equation 11. The sensitivity analysis conducted in this study demonstrates that PLA-based composites experience a more severe decline in their MCDM scores compared to epoxy-based composites under realistic defect conditions. The inherent polarity mismatch between hydrophobic PLA and hydrophilic natural fibers results in substantially lower k_adh values for PLA composites versus for epoxy composites, leading to reduced effective fiber volume fraction as described by Equation 12. This differential impact widens the performance gap between epoxy and PLA matrices beyond what is predicted by ideal models, with the sensitivity index for epoxy materials ranging from 67.5% to 74.0% compared to 49.1% to 53.2% for PLA materials. Notably, Ramie/PLA exhibits the lowest sensitivity index (49.1%) among all materials, indicating that despite its lower absolute performance, it retains relative performance better than epoxy composite under defect-prone conditions. Consequently, the ranking becomes more sensitive to fiber-matrix compatibility, favoring materials with higher k_adh values. From a design perspective, Ramie/Epoxy offers the highest absolute performance for defect-critical applications, while Flax/Epoxy provides the best balance of structural reliability and defect tolerance. Ramie/PLA emerges as the leading fully biodegradable option under realistic defect conditions, surpassing Flax/PLA in both absolute MCDM (0.759 vs 0.715) and sensitivity index (49.1% vs 52.7%).

4.6. Long-term durability in humid coastal desert environments

In humid coastal desert environments such as Neom, where relative humidity can fluctuate significantly and occasional fog or dew events occur despite the arid climate, natural fiber composites face long-term dimensional stability and stiffness degradation. When natural fibers absorb moisture, the cellulose microfibrils swell, inducing micro-cracking at the fiber-matrix interface, reducing the adhesion coefficient k_adh, and increasing the interphase thickness Δr_i as described in Equation 16. Over extended exposure, this leads to reductions in both longitudinal and transverse moduli. For Ramie/Epoxy and Flax/Epoxy, equilibrium moisture absorption typically low by weight, resulting in moderate reductions in tensile strength and modulus. For PLA-based composites, the situation is more severe: PLA itself undergoes hydrolytic degradation in the presence of moisture, accelerating property loss and potentially reducing service life by half in persistently humid environments. Ramie/PLA and Flax/PLA are particularly vulnerable due to their lower k_adh values, which exacerbate moisture-induced debonding. Several mitigation strategies exist. Protective coatings, such as polyurethane or acrylic-based sealants, can reduce moisture ingress, preserving mechanical properties for extended periods [³⁴].

From an environmental perspective, protective coatings add mass and introduce synthetic materials, partially offsetting the sustainability gains of natural fibers. However, the extended service life and reduced replacement frequency may justify this trade-off for long-endurance UAV operations.

4.7. Transition from microscale predictions to full wing structures

When moving from microscale predictions to full wing structures, several sources of deviation appear, as validated by the Halpin-Tsai model selection based on JARIWALA and JAIN [²⁹]. At the microscale, errors arise from fiber irregularities such as non-uniform cross-sections, internal voids (lumens), waviness, and the inherent variability in natural fiber properties due to growth conditions, harvesting time, and extraction methods. For ramie and flax fibers, which have fewer uniform cross-sections than synthetic fibers, noticeable deviations may occur in lamina elastic modulus predictions. At the mesoscale (lamina level), further variation is introduced by uneven fiber dispersion, resin-rich regions, and manufacturing porosity that commonly appears in natural fiber composites. At the macroscale (full wing structure), additional effects become important, including edge effects, stress concentrations near geometric discontinuities such as the wing-fuselage connection and control surface joints, as well as the accumulation of defects across larger structural areas.

4.8. Balancing performance enhancement with biodegradability

From a materials science standpoint, a composite is fully biodegradable only if all constituents fiber, matrix, filler, and surface treatments degrade into natural products such as water, carbon dioxide, and biomass within a reasonable time under relevant conditions. The addition of synthetic nanoparticles, such as carbon nanotubes or nanoclays, or chemically modified natural fillers like silane-treated cellulose nanocrystals, can leave non-biodegradable residues or by-products. Likewise, chemical surface treatments that improve fiber-matrix adhesion may introduce molecules that do not degrade. Based on the findings of this study, Ramie/PLA and Flax/PLA represent the best fully biodegradable options, with base MCDM scores of 0.759 and 0.715 respectively. However, any attempt to enhance their performance through synthetic additives would compromise their biodegradability claim.

Based on the work of CHEE et al. [³⁴], thermogravimetric analysis shows that natural fiber epoxy composites exhibit higher thermal stability than neat epoxy, with initial decomposition temperatures ranging from 282°C to 348°C, supporting fire resistance and structural integrity under thermal stress. Dynamic mechanical analysis also indicates that these composites achieve high complex and storage moduli at both room and elevated temperatures, which is critical for UAV wings exposed to solar heating in regions such as Tabuk and Neom, where surface temperatures can exceed 50°C.

Although epoxy has a higher environmental cost during production than PLA, its longer service life, higher thermal stability, and improved mechanical performance enhance sustainability through durability and reduced replacement frequency [³⁵].

A full life cycle assessment should balance these benefits against the initial environmental impact. Epoxy-based composites show lower moisture sensitivity and better fatigue resistance than PLA-based composites [³⁶, ³⁷]. These properties extend operational life and reduce maintenance and replacement needs, lowering material consumption over time. Ramie/PLA offers the best biodegradable alternative with outstanding specific stiffness (37.1 GPa·cm³/g) and the lowest sensitivity index (49.1%), making it a viable choice for environmentally driven designs where maximum stiffness is required.

When these factors are considered, the total environmental impact of Ramie/Epoxy and Flax/Epoxy can be similar to, or even lower than, biodegradable composites that wear out faster and need more frequent replacement, especially in long-endurance UAV applications.

5. CONCLUSIONS

This study applied a two steps Multi Criteria Decision Making approach to identify the most suitable sustainable composite materials for UAV wing structures. The first Base MCDM results indicate that Flax/Epoxy provides the best balance between mechanical performance and sustainability. It delivers high tensile strength of 529 MPa and a modulus of 42.5 GPa while maintaining strong specific properties and structural reliability. For applications where full biodegradability is required, Flax/PLA represents the most viable alternative. It offers tensile strength of 503.5 MPa and modulus of 31.2 GPa, with a moderate performance reduction compared to epoxy composites but clear environmental advantages due to its renewable fiber and biodegradable matrix.

The second MCDM approach integrated with an adhesion-based micromechanical model to identify the most suitable sustainable composite materials for UAV wing structures under realistic aerodynamic loading conditions. The analysis evaluated natural and fiber-reinforced composites with epoxy and PLA matrices, considering not only static mechanical properties but also sensitivity to gust factor and turbulence intensity.

The results establish a clear performance hierarchy. Ramie/Epoxy achieves the highest overall MCDM score (0.855) due to its exceptional specific stiffness (43.7 GPa·cm³/g) and superior flutter margin (1.20 relative to Flax/Epoxy), making it the optimal choice for stiffness-critical and flutter-prone wing designs. However, Flax/Epoxy provides the best balance between mechanical performance and sustainability, delivering high tensile strength (529 MPa), excellent adhesion, and a well-rounded property profile (base MCDM: 0.811) that suits general UAV applications. Hemp/Epoxy exhibits the highest tensile strength (549 MPa), recommending it for primary load-bearing components such as wing spars.

For applications where full biodegradability is required, Ramie/PLA emerges as the most viable alternative, achieving a base MCDM of 0.759 with outstanding specific stiffness (37.1 GPa·cm³/g) and the lowest sensitivity index (49.1%) among all materials, indicating superior relative retention of performance under gust and turbulence. Flax/PLA follows as a balanced biodegradable option (base MCDM: 0.715, strength: 503.5 MPa). The final material selection depends on design priorities: Ramie/Epoxy for maximum stiffness and flutter resistance, Flax/Epoxy for the best overall balance, Hemp/Epoxy for highest strength, and Ramie/PLA for optimal biodegradable performance.

Future research should focus on improving the mechanical performance of fully biodegradable composites to further reduce the gap with epoxy-based composites. Efforts should target enhanced fiber surface treatments to increase the adhesion coefficient (k_adh) of PLA composites, hybrid natural fiber reinforcement combining high-stiffness fibers like Ramie with ductile fibers like flax, and nano-scale fillers to increase stiffness and strength without increasing density.

Future experimental work should include laminate testing, dynamic mechanical analysis, long-term fatigue evaluation, moisture absorption studies, and environmental ageing assessments under realistic UAV operating conditions. Attention should be given to harsh hot-humid coastal desert environments such as NEOM, where elevated temperature, humidity, and saline exposure may significantly influence composite durability and structural integrity.

In addition, comprehensive life cycle assessment (LCA) and full-scale structural validation are required to confirm both sustainability and operational performance. These investigations should include aeroelastic flutter analysis, gust load simulations, and full UAV wing testing under representative flight conditions.

6. DATA AVAILABILITY

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

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Publication Dates

• Publication in this collection
  17 July 2026
• Date of issue
  2026

History

• Received
  24 Feb 2026
• Accepted
  22 May 2026